1. Field of the Invention
The present invention relates to a temperature compensation circuit performing a temperature compensation with respect to a gain characteristic of a variable gain amplifier fabricated by MOS transistors and gain-controlled by an external gain control signal, and a variable gain amplification circuit using the same temperature compensation circuit.
2. Description of the Related Art
In a conventional art, a variable gain amplifier has been developed so that a gain varies with an exponential function in respect to a gain control signal Vc. For example, U.S. Pat. No. 6,215,989 B1, the entire contents of which are incorporated herein by reference, a variable gain amplifier fabricated by bipolar transistors having an input/output characteristic representing an exponential function.
In late years, a variable gain amplifier using MOS transistors which is beneficial to cost reduction is developed. However, since the MOS transistor has an input/output characteristic of square-law characteristic, a circuit must be improved to obtain an exponential function characteristic.
The applicant provides a technique of realizing a variable gain amplifier utilizing a fact that the MOS transistor operating in a weak inversion region has an exponential-law characteristic expressed by an expression (4) (refer to the U.S. patent application Ser. No. 09/950,630, the entire contents of which are incorporated herein by reference).
However, since a thermal voltage W(=k/T/q) of the equation (4) is a variable proportional to the temperature T, a gain varies by the temperature as shown in FIG. 10. Therefore, this temperature variation must be suppressed.